Method of applying metallic coatings



1961 L. n. EUBANK EI'AL 2,994,951

METHOD OF APPLYING METALLIC COATINGS Filed Feb. 7, 1946 gwvem bom John WFab/0300 Lon/e .0, Eubank United States Patent 2,994,951 METHOD OFAPPLYING NIETALLIC COATINGS 7 Lowell D. Eubank, South-Euclid, and JohnW. Robinson,

Brecksville, Ohio, assignors to the United States of America asrepresented by the United States Atomic Energy Commission Filed Feb. 7,1946, Ser. No. 646,168 6 Claims. (Cl. 29- 529) insufficiently thick toprovide adequate protection for the base metal. Thus in the fission ofuranium in a neutronic reactor it has been found that zinc or aluminumsilicon alloy coatings may afiord protection for the uranium but thedurability of the coatings and their consequent life in use is muchshorter than desirable. Coatings of such metals may protect the basemetal for varying period depending upon the nature of the coating andthe thickness of the coating. Thus under conditions of corrosion whichprevail in a water-cooled reactor, an aluminum silicon alloy coating ofapproximately eutectic composition on the order of five to ten mils inthickness may provide adequate protection for the base metal for a shortperiod. However for longer periods the hot dip coating alone isinadequate to afford continuing protection for the underlying metal.

An object of the present invention is to provide heavy coatings ofprotective metals for various base metals. A further object of theinvention is to provide a new process for the application of metalliccoatings whereby substantially increased thicknesses may be secured. Afurther object of the invention is to provide heavy metallic coatingsfor uranium rods, such as employed in neutronic reactors. Furtherobjects of the invention will become apparent from a consideration ofthe following description of the invention and detailed discussion ofspecific examples illustrating several preferred applications of theinvention.

In accordance with the present invention a cylindrical object to becoated is heated to coating temperature and placed upon rotating rollersso that the rollers bear evenly against the surface of the metal object,and coating metal is then poured onto the rollers so as to form a poolin the valley between one of the rotating rollers and the cylindricalobject to be coated. The rotation of the rollers is continued until thecoating has been built up to the desired thickness. It has been foundthat by applying molten metal coatings to cylindrical objects in thedescribed man ner the thickness of the coatings is progressivelyincreased much in the manner that a snowball increases in size but eachlayer of metal is firmly bonded to the underlying layer so as to providean integral protective case of the protective metal.

The object to be coated should be heated to coating temperature prior torollercoating. Preheating may be accomplished by dipping the object intoa hot molten flux or into a non-reactive liquid, such as a liquidsilicone polymenat high temperature or into a bath of coating metal. Thelast method is norm-ally preferred since it applies a preliminarycoating which facilitates wetting of the object with the coating metalapplied on the rolls.

The application of the invention is not limited to any particular basemetal or coating metal but may be used wherever it is desired to buildup a coating heavier than obtainable by hot dip coating alone. Theprocess is capable of building up a variety of coatings on coated aswell as uncoated base metal. Thus a base metal which has been coated byhot dipping or elecroplating with one metal may be further coated by themethod of the invention with a different coating metal. The invention isof special importance in the coating of uranium for use in neutronicreactors since coatings applied by conventional methods are inadequatefor this purpose.

The rollers employed for carrying out the invention may be composed ofany material which does not adhere to the hot coating metal or the hotbase metal and which is not injured by the high temperature employed inthe coating process. Rollers constructed of carbon and of asbestoscement composition have been employed satisfactorily. The size of therollers is not critical, but to avoid excessive undulations in thecoating, they are preferably considerably greater in diameter than theobject to be coated. The rollers may be operated manually orautomatically. For application of uniform coatings to a large number ofidentical objects, automatic rotation is preferred. However for theapplication of coatings to objects where the conditions of applicationare varied, as in the case where objects of different sizes are to becoated or protective coating of different compositions or temperaturesare to be applied, manual operation of the rollers permits moreversatile control to suit the particular conditions involved. Speeds ofrotation may vary to provide peripheral velocities from 10 to 300 ormore feet per minute. In general the rate of rotation should be suchthat the surface of the object as it contacts the molten metal is closeto or slightly below the melting point of the molten metal. This permitsrapid wetting of the surface with the molten metal and addition ofmolten metal to the surface at a rate greater than run oil of moltenmetal from the surface so that the desired accretion of metal on thesurface is obtained.

The rollers should be kept free from accumulations of solid metalparticles. This may be accomplished by frequent wiping of the rollers orby provision of brushes or swabs of material such as fibre-glass,asbestos, or siliconepolymers which are not seriously affected by thehigh temperatures involved.

The rollers may be heated internally if desired. However, the processmay be conducted satisfactorily with rollers provided with no heating orcooling means other than exposure of the surface to normal atmosphericconditions. With some objects, such as hollow cylinders, it may bedesirable to provide heating means for the forward roller and coolingmeans or no temperature regulation for the rear roller. For very largeobjects it may be desirable to provide cooling means for the rear rollerwith or Without heating means for the forward roller. The entire processmay be conducted in an atmosphere of controlled temperature andcomposition with or without application of radiant heat to the object orthe coating metal. Conditions which assure constant radiation heatlosses are desirable Where a maximum uniformity of coatings is to beobtained. However, remarkably successful results have been achievedwithout any regulation of these factors, the heat supply beingcontrolled merely by regulating the initial temperatures of the coatingmetal and the object to be coated.

During the coating procedure the metal may tend t over-run the objectbeing coated and thus form a lip of hand-operated apparatus.

- acter of the coatings at the ends of rods or tubes is better when'theradius of curvature of the junction ofthe end faces with the cylindricalface is-large than when it is small or substantially non-existent. Sincethe process does not coat the ends of cylindrical objects, it ispreferred to carry out the process upon objects which have beenpreviously treated to protect the ends. One highly satisfactory methodfor accomplishing this purpose is to provide caps or ferrules on theends of the object prior to coating in accordance With the invention.

The invention illustrated in the accompanying drawing showingapplication of coatings by means of a simple apparatus comprises frame 1which supports pivoted roller rack 2. The rack 2' is shown held at a 45angle to the horizontal by a pair of adjustable brackets 3. This hasbeen found to be a'convenient working angle. The rack 2 carries twospaced asbestos cement composition rollers 4 and 5. Roller 4 is drivendirectly by a crank handle 6 and roller indirectly through the chainlinkage 7 so that its surface speed is the same as that of roller 4. Adistributor 8 comprising a sheet of asbestos cement composition or otherhard, heat-resistant material is provided in front of the forward roller4 and supported on adjustable brackets 9 so that its position withrespect to roller 4 can be regulated. It has been found convenient toset this'distributor so that its upper surface is inclined a fewdegrees, say about 5 degrees, from the horizontal down toward roller 4and its upper edge is substantially contiguous to roller 4. By pouringmolten metal onto the distributor 8 a more even distribution of themetal along the valley between roller 4 and the article to be coated isobtained than when an attempt is made topour the metal "directly'intothis valley. Brushes '10 and 11 are held by springs 13 against therollers 4 and 5. "These brushes may be wire, fibre-glass, asbestosfibre, or other heat-resistant material. The brush holders are held inposition by bifurcated ends which form guideways bearing against theends 'of rack 2.

For applying coatings by means of this equipment the article to becoated, represented as a rod 14, is preheated,

preferably by dipping in a molten bath of the coating metal and isplaced in'the valley between rollers 4 and 5.' The rollers are rotatedby hand-crank 6 at the desired rate of rotation and additional coatingmetal is poured onto the distributor 8 from which it flows as a sheet 15into the valley between rollers 4 and 5. This sheet or pool hassubstantial thickness and the weight of the rod 5 14 depresses the sheetso that it piles up somewhat immediately in front of rod 14 to form awave 15' of molten metal in the larger pool 15. It is preferred to pourout a measured quantity of the coating metal and to rotate the rollers 4and 5 until all of the coating metal has been taken up by the rod 14andthe coating has solidified.

This procedure irregularities that may result from the wave of moltenmetal in front of the rod if an attempt is made to remove the rod beforethis wave has been used up. The rod is then cooled and quenched ifdesired and subsequently trimmed, machined, and

burnished to provide the final finish. Normally it is desirable to applya coating about 15 mils thicker than the thickness ofthe desired coatingand to machine off the excess metal. Thisprocedure eliminatesirregularities in the surface o-f the coating, 'and'the .finalburnishing pro- -vides a-smooth worked metal surface which appears topossess superior corrosion resistance.

The process of theinvention hasbeen found to be par- -tic ul-ar1ysuitable for theapplication of 'aluminumsilicon coatings to metallicuranium rods. The metallic uranium is preferably provided with a.blockingcoatto prevent reaction of the uranium metal with thealuminum-silicon alloy to form a relatively brittle intermediatecompound layer. The blocking coat may be a copper alley or anelectroplated chromium layer. Electroplated copper, iron and nickelblocking layers and hot dip coatings of zinc, and brass also have beenemployed with some degree of success. In additionto the aluminum-siliconalloy coatings, cojatings 'ofpure'aluminum and coatings of purezinc-aluminum alloy also have been ap lied by the process of theinvention. aluminum-silicon alloy coating having approximately thecomposition of the eutectic is employed, thecharacter of the coating issubstantially improved by'the presence of a small proportion of sodiumor zinc. Thorough degassification also is beneficial for the productionof sound aluminum-silicon alloy coatings. 7

The following examples illustrate the application of the invention tothe preparation of metallic rods for use in a neutronic reactor.Quantities. are expressed in terms of weight if not otherwise indicated.

Example 1 A uranium rod about 8 inches long by 1.36 inches in diameterwas prepared for coating by dipping in aqueous 60% HNO solution at 65 C.for about 5 minutes, rinsing, and drying. The rod was promptly dippedina bronze bath comprising 47 parts copperand 53 parts tin at atemperature of 730 C. for 20 seconds, during which the bath cooled to680 C. The rod was removed, centrifuged 5 seconds to remove excessbronze, and dipped for 20 seconds into a molten salt bath comprising 53%KCl, .42% U01, 5%. at 600 C. The rod was next dipped for 20 seconds intoan aluminum-siliconalloy. bath-comprising 88 parts of aluminum and 12partsof silicon modified by 0.1% sodium, maintained at 640 C. IUponwithdrawal from this bath. the rod was placedbetween Example 2 Ametallic uranium rod prepared as described in Example 1 was dipped for 1minute in a 47% copper 53% tin bath at a temperature falling from 700":to 680 C. during the dipping period. The bronze bath was protected byabout /z-inch of 53% KCl, 42% M01, 5% 'NaCl flux. Upon withdrawal fromthe bronze bath, the rod was centrifuged 5 seconds and plunged into asalt bath of the same composition as the flux for 20 seconds at 600 C.The rod was then dipped for 10 seconds in an unmodified aluminum-siliconaHoy bath containing 88 parts of alumi- -num and 12 parts of silicon at640 C. Upon withdrawal from the aluminum-silicon bath the rod wasplacedon the asbestos cement composition rollers and 23 grams of thesame aluminum-silicon alloy and then 35 grams of alloy comprising 88parts of aluminum, 12 parts of silicon and 1% of zinc and 0.02% ofsodium both at 640 C.

were poured onto the forward roller. Rolling required silicon pouredonto the rollers.

approximately 15 seconds to apply all of the aluminum The rod was thenremoved from the rollers and plunged intowater to quench the coating.After quenching, the coating was trimmed,

- machined and buffed to asmooth even finishtoprovide a 30-mil coating.

Example 3 The preferred procedure comprises dipping the uranium rodthrough a flux of 53% potassium chloride, 42% lithium chloride, sodiumchloride into a molten bronze bath consisting of 53 parts of tin and 47parts of copper at a temperature of 720 C. for 45 seconds. The rod isremoved from the bronze bath and promptly immersed in a bath of moltentin at 640 C. for 20 seconds. Upon removal from the tin bath the rod iscentrifuged at 500- 5000 r.p.m. for about 5 seconds to remove excess tinand the ends immersed in a bath of 0.l%-sodiummodified 13X aluminumsilicon alloy (Federal Specification AN- QQ-A-366, amendment 4 Al-lSX)degassed by passing chlorine diluted with nitrogen through the alloyprior to introduction of the sodium. After each end of the rod has beenin this bath for about 2 seconds, it is removed and aluminum caps arepressed over the ends of the rod by means of expandable dies which forma smooth fitting cap welded to the rod by the molten aluminum-siliconalloy. The rod is then again dipped in the aluminumsilicon alloy for twoseconds, withdrawn, placed upon the asbestos cement composition rollersand rotated while about 33 grams of additional aluminum-silicon alloywhich has been degassed and sodium-modified and is at a temperature of800 C. is poured onto the forward roller. Rolling is etfected at about130 to 140 r.p.m. until the coating solidifies, which requires aboutseconds. The coating is then quenched, trimmed, machined and polished aspreviously described.

Example 4 A metallic uranium rod about one inch in diameter was cleanedas described in Example 1, then dipped through a 53% KCl, 42% LiCl, 5%NaCl flux into a 47% copper, 53% tin bath at 740 C. and held there forseconds. It was next immersed in molten tin at 400 C. for 30 seconds,withdrawn and centrifuged for 5 seconds in air. The centrifuged rod wasdipped in an 88% aluminum 12% silicon bath at 640 C. for 3 seconds andthen placed immediately on asbestos cement composition rollers androlled for 5 seconds while 40 grams of 2 S aluminum at 680 C. was added.The coated rod was promptly quenched by pouring water over it and whencool was machined to a coating thickness of 30 mils.

Example 5 A metallic uranium rod about 1 inch in diameter and 4 incheslong having a chromium electroplating 0.0008 inch thick thereon wasdipped through a potassium, lithium, sodium chloride flux of thecomposition employed in Example 4 to which 10% sodium chloride had beenadded into a zinc bath containing 1% of aluminum maintained at atemperature of about 650 C. After one minute in the bath, the rod waswithdrawn, centrifuged to remove excess molten metal and dipped into a88% aluminum, 12% silicon bath at 640 C. for 4 seconds. The rod waswithdrawn from the aluminum silicon alloy bath and placed on asbestoscement composition rollers and rolled while 35 grams of additional 88/12 aluminum silicon alloy at a temperature of 640 C. was rolled on.

Example 6 A copper electroplated uranium rod about one inch in diameterand 4 inches long was dipped into a 47% copper, 53% tin alloy bath at718 C. for 30 seconds. The rod was withdrawn from this bath and immersedin molten tin at 500 C. for 5 seconds. It was then removed from the tinbath and immersed in an 88% aluminum, 12% silicon bath at 640 C. for 2seconds. Upon withdrawal from the aluminum silicon bath it was placed onasbestos cement composition rollers and 50 grams of additional aluminumsilicon alloy was rolled on. Rolling was continued until the coatingsolidified and the rod was then plunged into water to cool the coating.A thick, firmly adherent coating was thus obtained.

Example 7 A one inch diameter by four inch long metallic uranium rodprepared as described in Example 1, was dipped through a 53% KCl, 42%U01, 5% NaCl flux in a bath of 99% zinc, 1% aluminum at 480-500 C. for 5minutes, withdrawn, centrifuged at 600 rpm. 5 seconds then dipped inafluxless bath of tin, 10% zinc at 325-335 C. for one minute. The coatedrod was placed on asbestos cement composition rollers and rolled at rpm.for about 30 seconds while an alloy of 90% tin 10% zinc at about 325 C.was poured on. The rod was then quenched in water. The total increase inweight of the rod was 8 grams.

It will be understood that we intend to include variations andmodifications of the invention and that the preceding examples areillustrations only and in no wise to be construed as limitations uponthe invention, the scope of which is defined in the appended claims,wherein we claim:

1. The method of coating a cylindrical metal object, which comprisessupporting the cylindrical object between a pair of rollers, maintaininga pool of coating metal between the object and one of the rollers, androtating the rollers to bring the surface of the object repeatedly intocontact with the pool of coating metal, thereby progressively increasingthe thickness of the coating.

2. The method of coating a cylindrical metal object, which comprises,supporting the object upon a pair of rollers, pouring molten coatingmetal between the object and one of the rollers at a higher temperaturethan said object, and rotating the rollers to bring the surface of theobject repeatedly into contact with the molten coating metal, therebyprogressively increasing the thickness of the coating.

3. The method of coating a cylindrical metal object, which comprisesdipping the object into a molten metal bath to cover its surface with auniform thin layer of coating metal, placing the object upon a pair ofrollers, flowing molten coating metal between the object and one of therollers, and rotating the rollers to bring the surface of the objectrepeatedly into contact with said coating metal, thereby progressivelyincreasing the thickness of the coating.

4. The method of providing a protective coating on a metallic uraniumrod, which comprises applying to the uranium rod a metallic barriercoating and then dipping the rod into a molten aluminum silicon alloybath having approximately the composition of the eutectic to provide auniform coating of the alloy over the barrier coating at least on theends of the rod, applying aluminum and protectors to the ends of therod, again dipping the rod into molten aluminum silicon alloy havingapproximately the composition of the eutectic, placing the rod betweenand on a pair of spaced rollers, maintaining a pool of coating metalbetween the rod and one of the rollers, and rotating the rollers tobring the surface of the rod repeatedly into contact with said pool ofcoating metal, thereby progressively increasing the thickness of thecoating, then promptly quenching the coating, and subsequently machiningand burnishing it to provide a smooth corrosionresistaut surface.

5. The method of coating a cylindrical uranium object which comprisesdipping said object into a molten metal bath to cover its surface with auniform thin layer of metal, placing said object upon a pair of rollers,pouring a molten coating metal between said object and one of saidrollers, and rotating the rollers to bring the surface of the objectrepeatedly into contact with said molten coating metal, therebyprogressively increasing the thickness of the coating.

6. The method of coating a cylindrical uranium object which comprisesapplying to said object a metallic barrier coating, dipping said objectinto a molten aluminum silicon alloy bath having approximately thecomposition 7 of the eutectic, placing saidobject between and on a pairof spaced rollers, maintaining a pool of coating metal 'f'betweens'aidobject and one of said follers, and rotating fthenollers to bringlthesurface of said 'object repeatedly into contact with said pool ofcoating metal, thereby 5 progressivelyincreasing, the thickness of thecoating.

UNITED STATES, PATENTS Becht. --'1M3I:,22;1"19i27 Steckel May- 9, m3Castle Nov. 21, 1933 1 Waltman Mar. 22, 1938

